How cloudy is it today? Is it going to rain? The answer might depend on how much pollution is wafting around in the air above you. Two recent studies published in the journal Science show how particles pumped out of power stations and car exhausts alter weather. The first study, by Antony Clarke and Vladimir Kapustin from the University of Hawaii, analysed pollution levels over different areas of the Pacific ocean over the last 11 years. In the more polluted regions (such as parts of the north Pacific) there were more than 10 times as many particles on which cloud water could condense than there were in pristine regions (such as above Tasmania).

A second study, by Scot Martin and colleagues from Harvard, looked at particles over a remote area of the Amazonian basin in Brazil, during the rainy season, when the air is at its most pristine, similar to pre-industrial conditions. They found that low pollution conditions encouraged the formation of convective clouds and frequent rainfall. But when pollution increased, the higher concentration of particles created clouds with smaller droplets, which produced less frequent, but more intense, rain.

It is not always the polluters who pay, since pollution can travel for thousands of miles, across oceans and over mountains. Next time that it is dull and dry, or a flash-flood hits, you could blame the weather on the activities of those living upwind.